Carburetors are widely used to produce and control the mixture of fuel and air delivered to an operating engine. Current carburetors utilize a needle valve assembly to meter the quantity of fuel in the fuel and air mixture. The needle valve assembly comprises a pin or needle threadably received in a bore of the carburetor and rotatable to vary the location of a conical end of the needle relative to a machined annular seat to control the flow area between the needle and the seat and thereby control the amount of fuel which flows therethrough. One of the major problems with needle valve assemblies is that a fuel flow change can and usually does occur after the needle valve assembly has been adjusted. This fuel flow change is caused by axial and radial movement after adjustment of the conical tip of the needle relative to the seat which may be caused by vibration, temperature changes, installation of limiting caps and other physical side loading of the needle. Radial movement of the needle relative to the valve seat decreases the gap between the needle and the valve seat adjacent one portion of the needle and increases the gap on the opposite portion of the needle which can drastically affect the fuel flow characteristics therethrough.
Another problem with the needle valve assemblies is the size of the metering orifice. The annular fuel flow area between the needle and the valve seat is generally on the order of about 0.001 inches to 0.002 inches wide. Most particles such as dirt or aluminum flakes within the carburetor are too large to pass through this gap and may at least partially clog the fuel flow area causing the engine to run leaner than desired.
Additionally, the construction and arrangement of a needle valve assembly is determined by the calibration requirements of each individual engine family. Therefore, the needle valve assemblies are generally not interchangeable among the various engine families resulting in the need for hundreds of different mixture needle valve assemblies to accommodate the various carburetor models and engine designs. Still further, variances in the production of the needle and the valve seat result in an inconsistent relationship between the needle and the valve seat which causes carburetors with the same calibration requirements to have inconsistent fuel flow rates.
Also, to limit the extent to which the end user could vary the fuel flow rate through a needle valve assembly a limiter cap or the like has to be installed on the needle to limit the extent to which it can be rotated. These limiter caps increase the cost to manufacture and assemble the needle valve assemblies and may cause the needle valve to shift relative to its seat as the caps are installed and thereby, after adjustment, alter the fuel flow rate of the valve assembly.